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Muddying the Gene Pool
Cattle connections going back a hundred years have caused a threat to plains bison
Story by Ben Singer

Recent evidence of cattle genes in the DNA of North American plains bison herds has conservation scientists raising an alarm. In a May 2004 report, the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) listed the plains bison as "threatened" for the first time. It cited evidence of hybridization with cattle — and fears this genetic impurity may get worse — alongside habitat and disease considerations.

While about 700,000 bison live in North America today, just five percent belong to wild herds. In Canada that amounts to four wild or semi-wild herds totalling about 1,000 individual bison. Although no cattle genes have been found in Canada’s wild herds — at Pink Mountain, B.C.; Elk Island National Park, Alta.; and Saskatchewan’s Prince Albert National Park and Cold Lake/Primrose Air Weapons Range — the COSEWIC report says that importing animals from impure herds in the United States and close contact with private bison farms pose a significant threat to the species’ genetic integrity.

Other protected plants and animals, such as the Florida panther, have had conservation problems after mingling with related species. In the panther’s case, an escaped South American puma caused a line of hybrids, and much hand-wringing over whether a hybrid animal deserves endangered status.

Tracking the errant genes

With bison, the story starts in the late 1800s, when ranchers began rounding up the few remaining plains bison in order to establish herds. Some crossed the bison with cattle. Animals donated by those ranchers provided the genetic stock of today’s public herds - cow genes and all.

In 1994, biologist Curtis Strobeck and his colleagues at the University of Alberta took DNA samples from a herd at South Dakota’s Custer State Park, hoping to create a better bison family tree. What they found surprised them.

Strobeck recalls the night he noticed some unusual DNA data coming back from sequenced animals. "I looked at it in the morning and saw that it was very different and compared it to cattle, and it matched perfectly," he remembers.

The group examined mitochondrial DNA, a genetic material passed directly from mothers to daughters. Tracing the cattle gene back to the founding of the park, Strobeck and his colleagues deduced that the culprit was likely a female bison with one-quarter domestic blood.

Although the Canadians found cattle genes in only two of 32 Custer bison tested, in 1999 U.S. researchers found that more than 20 percent of the Custer bison had cattle mitochondrial DNA, as did five other public herds in the U.S.

In those studies and others, the four Canadian wild herds showed no signs of cattle genes. Strobeck says that is because those herds were stocked largely with animals from Elk Island National Park. Private Canadian ranches, however, often get animals from Custer State Park and other U.S. parks with known cattle genes, he says.

Elk Island National Park wildlife biologist Norm Cool says he is "almost 100 percent certain that we don’t have cattle genes of any type in our bison population." He credits the herd’s good records, genetic tests, and a seven-foot-high fence that surrounds the park and prevents stray ranch animals from getting in.

Cool says that once COSEWIC’s report is adopted, emphasis will shift toward maintaining purity in plains bison herds, giving Elk Island an even more prominent role. While the park often sells excess animals to bison ranchers, Cool says commercial operations must be considered separately from conservation efforts, but adds, "It isn’t out of the picture to maybe discuss with the game-ranching industry, to develop some kind of agreement with a pure-bred registry system."

While Strobeck says that most private ranchers in Canada have some connection to genetically suspect Custer State Park bison, Mark Silzer, a rancher and president of the Canadian Bison Association, says he isn’t convinced there is a threat to bison or to ranchers’ livelihoods.

Without any obvious difference in bison with cattle genes, it would be impractical to identify hybrid animals using current technology, he says. "The only way you could find out for sure would be to go and test the 500,000 head in North America, and find out how many of them are pure, and obviously nobody’s going to do that."

Strobeck says that while nobody wants to slaughter all bison with connections to Custer State Park, efforts should be made to use pure bison as much as possible. "The best thing to do is to take the herds that are knowingly or seemingly pure," he says. "There’s no indication that you could ever prove that it’s absolutely pure, but the plains bison of Elk Island National Park and Yellowstone (National Park) bison, those seem to be pure."

If the COSEWIC recommendations pass public consultation, plains bison will be added to the threatened list of the Species at Risk act, and, for the first time, federal measures will be put in place to protect the purity of these prairie sentinels.

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